Immersion type nickel stripper



EXAMINER Jan. 23, 1968 E. B. SAUBESTRE ET AL 3,365,401

IMMERSION TYPE NICKEL STRIPPER Filed March 14, 1967 R 5 3 Aim 7 o e O Om Om Ow Ow Ow Om ON Q M v m w s 0 u M m u H v PM a W r T 2: SEE $2 H M o $15 llllqfighw 200x523 12: B [I I f: LEI EZCSQ W 1! ON \0 n DE Amfidz mam yiigguv m m EEC wd 53w; 23% m m Y W B L N @Efifis: Q 1 Wm $864128 $052022: H55 M W wm m m m a m w m w United States Patent 3,365,401 IMMERSION TYPE NICKEL S'I'RIPPER Edward B. Sanbestre, Hamden, Conn and Juan Hajdu,

Rocky River, Ohio, assignors to Enthone Incorporated, New Haven County, Com, a corporation of Connecticnt Continuation-impart of application Ser. No. 468,856,

July 1, 1965. This application Mar. 14, 1967, Ser.

Claims. (Cl. 252-149) ABSTRACT OF THE DISCLOSURE This invention relates to baths for stripping nickel from base metal objects or articles having nickel deposited thereon, to dry compositions adopted for dissolution in an aqueous solvent, for instance water, to form the nickelstripping baths, and to a method for stripping nickel from base metal objects or articles having nickel deposited thereon involving the contacting of such objects with the stripping bath or solution, usually by immersing the object in the bath. The nickel stripping baths herein comprise an aqueous solution of (l) A nitro-substituted mononuclear carbocyclic aromatic compound having at least one NO group attached to the aromatic ring and a water-solubilizing substituent also attached to the aromatic ring;

(2) A complexing agent for nickel ions selected from the group consisting of (a) a tribasic carboxylic acid of the formula wherein R and R are selected from the group consisting of CH and CHOH- and R is selected from the group consisting of and water-soluble salts thereof, (b) a dibasic carboxylic acid of the formula HOOC-(Cl-I -COOH wherein n is an integer from 1 to 4 inclusive, and water-soluble salts thereof, (c) a-hydroxy mono and di-carboxylic acids of the formula R,(R ),,CHOHCOOH wherein R is seelcted from the group consisting of H and COOH, R is selected from the group consisting of CH and -CHOH- and n is an integer from 0 to 4 inclusive, and water-soluble salts thereof, and (d) mixtures of at least two of (a), (b) and (c);

(3) Ammonium ions supplied as a water-soluble ammonium salt in amount sufiicient to maintain the pH of the bath within the range of from about 6.8-7 to up to but below a predetermined higher pH at which nickel ions substantially cease to be complexed by the carboxylic acid and instead are complexed by amomnia as indicated by a change in coloration of the bath from green to blue; and

(4) A sulfur compound yielding in the aqueous solution sulfur ions in a 2 oxidation state in a small amount sufiicient to accelerate the stripping of the nickel deposit from the base metal. The weight ratio of the nitro-substituted carbocyclic aromatic compound to the carboxylic acid complexing agent is between about 1 and about 1 to 4 respectively, the pH of the bath being within the pH range previously specified herein.

Cross-reference to a related application This is a continuation-in-part application of our copending US. patent application Ser. No. 468,856, filed July 1, 1965, now abandoned.

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Background of the invention (1) Field of the invention.-This invention relates to new and improved compositions and method for stripping a nickel deposit from a base metal object or article on which it is deposited, for instance a nickel-electroplated base metal object or article.

(2) Description of the prior art.Despite the use of the most modern techniques of electroplating, including use of automatic plating equipment, the most modern plating solutions, and the best in cleaning practice, a certain percentage of plated parts will prove to be defective after plating. Such defective parts may result from defects in the basis metal, improper cleaning, excessive porosity of the substrate which leads to bleeding out of various cleaning and plating solutions, impurities in the plating solution, etc. If the part which was defectively plated is sutiiciently valuable, it is economically desirable to strip the plated article of part or all of the plated coating and replate it.

The metal being stripped must be oxidized from the metallic state in which it is originally present to some sort of ionic state. Hence, the active ingredient in the stripping solution must be an oxidizing agent. Alternatively, the work being stripped may be made the anode in the stripping tank, so that the current which flows provides the required oxidizing action. In either case, there is a second, equally important requirement which the stripping solution must fulfill. The oxidizing agent, or anodic action, must be sufficiently strong to effect removal of the metal being stripped, by dissolution, yet must either not attack the basis metal, or must have a satisfactorily slow rate of attack on this metal. There are three basic ways in which such selectivity may be achieved between stripping action on the metal being removed and the basis metal:

(1) The metal to be stripped may be inherently much more active, electro-chemically, than the basis metal. For example, in the stripping of zinc from steel, there are a great number of solutions which may be used, because there are so many electrolytes in which zinc is more active than steel electrochemically.

(2) A complexing, chelating or clathrating agent may be used which is much more effective for the metal being stripped than for the substrate metal. Such agents will increase the activity of the ion of the metal being stripped so as to convert even relatively noble metals into relatively active ones. For example, in most electrolytes, gold is a very noble metal, yet, in the presence of an excess of the cyanide ion, it becomes a relatively active metal, and may be easily stripped from a variety of substrates.

(3) An inhibitor may be added to the stripping solution which preferentially reacts with (chemisorbs) or physically absorbs on the substrate. For example, many electrolytes and anodic stripping systems which will permit the rapid stripping of nickel will also tend to strip copper. If it is desired to strip nickel from a copper substrate in such a bath, it is necessary to add an inhibitor which will not interfere with the action of the stripper on nickel, but which will effectively adsorb or chemisorb on the copper substrate to inhibit the attack of the stripper on the copper. Organic or inorganic sulfur compounds are suitable for this application.

As previously noted, an oxidant is required to strip a metal; this oxidant may be either a chemical oxidizing agent present in the stripping solution (immersion stripping) or the use of electric current (anodic stripping). Both types of strippers are used, and each type has respective advantages and disadvantages. On the average, in both European and American commercial practice, immersion strippers tend to be more widely used than anodic strippers. The main reason for this seems to be that the proper operation of anodic strippers in a large commercial stripping operation is more difficult to achieve and maintain than in the case of immersion strippers.

More nickel is currently stripped than any other metal. In large measure, this is due to the wide popularity of nickel under chromium as a decorative, corrosion resistant coating on steel, copper, brass, aluminum, and zinc, and the widespread engineering applications for hard, corrosion resistant coatings. Another reason, however, is that automation in plating shops, and the use of highly leveling, bright nickels which require no butting or polishing, has made the nickel plating operation more critical than it was in earlier years, leading to the increased possibility of rejects which must be stripped.

Some of the earliest nickel strippers were immersion baths based on various combinations of strong inorganic acids. However, for many years the most popular type of nickel stripper has been the use of anodic sulfuric acid. Within the past ten years, however, immersion strippers have once again become very popular for nickel stripping. The modern immersion strippers are generally based on organic oxidizing agents, and are used in conjunction with alkaline solutions, for stripping nickel from steel, or acid solutions, used primarily for stripping nickel from brass and copper.

In the decorative plating of steel, nickel is sometimes plated directly over the steel, and sometimes over the copper undercoat. Therefore, depending on requirements, nickel strippers are sometimes designed to remove nickel without attack on either copper or steel, and sometimes to remove both nickel and copper coating at a rapid rate without attack on steel.

Immersion baths currently utilized for stripping nickel are largely based on organic oxidizing agents. A particularly useful oxidizing group is the nitro-group, NO of certain substituted nitrobenzenes. This grouping is rather easily reduced, with a resulting oxidation of the metal to be stripped. Although there are quantitative differences among them, a great many organic nitrocompounds have suitable oxidation potentials for the stripping of nickel. However, the solubility of the compound in the stripping solution is also important, since many nitro-organic compounds have only a limited solubility in water. Generally, it is necessary to attach one or more water-solubilizing substituents, i.e. solubilizing side chains, groups, radicals or atoms, to the aromatic ring of nitro-benzene. Such water-solubilizing substituents include carboxyl, COOH; aldehyde, CHO; sulfonic acid, +8O H; hydroxy, OH; chloro, -Cl; bromo, Br; etc. US. Patent 2,649,361 issued to Springer and Meyer, US. Patent 2,698,781 issued to Meyer, and U5. Patents 2,937,940 and 3,102,808 issued to Weisberg and Butler provide details on a number of suitable nitro-compounds. The following nitro-compounds are cited by these authors.

Aromatic:

m-, p-nitrochlorobenzene o-, m-, p-nitrobenzoic acid o-, m-, p-nitrobenzene sulfonic acid o-, m-, p-nitroaniline o-, m-, p-nitrophenol nitrophthalic acids 2-nitroresorcinol 2,4-dinitroresorcinol 2,4,6-trinitroresorcinol 0-, m-, p-nitrobenzaldehyde o-, m-, p-nitrotoluene Aliphatic:

1-, Z-nitropropane 1-, Z-nitrobutane dinitropropanes hydroxynitropropanes nitropentanes nitrourea tris (hydroxymethyl) nitromethane In the examples of the nitro group-substituted aromatic compounds set forth supra, the water-solubilizing substituent is attached to an aromatic ring carbon not linked or attached to a nitro group.

Although both nitro group-substituted mononuclear carbocyclic aromatic compounds and nitro group-substituted aliphatic compounds are set forth in the foregoing list, experience by and large has indicated that the nitroaromatic compounds are more valuable than the nitroaliphatics. The latter have but limited solubility in water, have limited stripping power if the molecular weight of the nitroaliphatic is too great, and are too volatile at low molecular weights.

For stripping nickel from steel objects, alkaline solutions have been found most useful inasmuch as acid solutions tend to etch or otherwise attack steel. However, since nickel forms an insoluble hydroxide in alkaline solutions which would prevent further stripping action, it is necessary also to add a complexing or chelating agent for nickel. Saubestre (Plating 45, 479-485 (1958)) has calculated the relative oxidation potentials of nickel in the presence of various complexing and chelating agents at a pH of 11, with the following results:

It is obvious from this listing that the most effective, lowcost complexing agent to use is cyanide, which forms the following complexes with nickel: Ni(CN) and Ni (CN) Also used commercially are the amines, which form the following type of chelate with nickel CHz-Nl'i: NIH-CH1 H Z i cmNin Nut-on In general, in commercial practice, the cyanide type of stripper is used to remove nickel from steel, stainless steel and magnesium, while the amine type of stripper is used to remove nickel from copper and brass, or nickel from parts which contain both steel and copper alloys. More recently, ammonia has come to be used as the complexing agent in nickel strippers for use on steel, copper, and brass. In such baths, the probable complexing species in Ni(NH For stripping nickel from steel, no further inhibitors are necessary. However, when stripping nickel from copper and brass inhibitors should be added. The most suitable types are the sulfur compounds characterized by yielding in the aqueous bath or solution sulfur ions in a 2 oxidation state. A listing of such sulfur compounds is given in US. Patent 3,102,808. Commercially, the most commonly used of these sulfur compounds are types III (1), III (2), IV (1), V (2), and V (3) listed in columns 1 and 2 of Patent 3,102,808.

The following are typical examples of the prior strippers of the cyanide, amine and ammonia types mentioned above:

Used at a bath temperature of 60 C.- C.

(c) Ammonia type:

Nitroaromatic grams/liter 35 NH OH(28% solution) milligrams/liter- 250 (NH SO grams/liter 100 Used at a bath temperature of 60 C.-80 C.

Where the sulfur compound inhibitor and accelerator has been used in the prior stripper compositions, typically about 0.1%- 2% by weight has been used of the sulfur compound.

The following comparisons may be made between these three types of prior art alkaline nitroaromatic strippers:

(1) The cyanide-type is much less expensive than the amine type, and comparable to the ammonia-type.

(2) The amine-type is the most stable at elevated temperatures, and the ammonia-type is the least stable due to volatility of the ammonia. Primarily for this reason, it is feasible to strip at higher rates in the amine-type baths.

(3) The amine-type bath has the least tendency to etch the substrate, the ammonia-type the greatest.

(4) The cyanide-type bath generally leaves the work in somewhat better condition for replating.

(5) When properly inhibited with thio-compounds, the amineand ammonia-type baths will not attack copper and brass, while the cyanide-type will, generally speaking.

(6) From the viewpoint of waste-disposal problems, the cyanide-type is the most objectionable, and the ammonia-type the least.

Summary of the invention In accordance with the present invention, new and improved stripping compositions are provided for stripping or selectively dissolving nickel deposited on a base metal object or article and which constitute a considerable improvement over the prior art cyanide-type, amine-type and ammonia-type stripping baths. The stripping bath of this invention comprises an aqueous solution of (1) A nitro-su-bstituted mononuclear carbocyclic aromatic compound having at least one --N0 group attached to a benzene ring and a bath compatible watersolubilizing substituent also attached to the benzene ring;

(2) a complexing agent for nickel ions selected from the group consisting of (a) a tribasic carboxylic acid of the formula wherein R and R are selected from the group consisting of -CH and --CHOH- and R is selected from the group consisting of l CH and C Onand water-soluble salts thereof, (b) a dibasic carboxylic acid of the formula HOOC-(CH --COOH wherein n is an integer from 1 to 4 inclusive, and water-soluble salts thereof, (c) a-hydroxy monoand di-carboxylic acids of the formula R (R -CHOHCOOH wherein R is selected from the group consisting of -H and -COOH, R is selected from the group consisting of --CH and --CHOH- and n is an integer from 0 to 4 inclusive, and water-soluble salts thereof, and (d) mixtures of at least two of (a), (b) and (c);

(3) Ammonium ions supplied as a water-soluble ammonium salt in amount sufficient to maintain the pH of the bath within the range of from about 6.8-7 to up to but below a predetermined higher pH value at which nickel ions substantially cease to be complexed by the carboxylic acid and instead are oomplexed by ammonia as indicated by a change in coloration of the bath from green to blue; and

(4) A small amount sufiicient to accelerate the stripping of nickel from the base metal object, of a sulfur compound yielding in the aqueous solution sulfur ions in a 2 oxidation state. The weight ratio of the nitro-substituted aromatic compound to the complexing agent herein is between about 1 (i.e. 1 to l) and about 1 to 4, respectively, and the pH of the bath is maintained within the pH range aforesaid. The stripping baths of the invention constitute a considerable improvement over the cyanide-type, amine-type and ammonia-type previously mentioned herein and of the prior art by reasons of:

(1) Not attacking substrates or bases of the steel, copper, brass and similar copper alloys, whereas the cyanide-type stripper attacks copper and brass substrates, and the ammonia-type stripper attacks steel substrates;

(2) Not presenting the waste disposal hazards of the cyanide and amine type strippers;

(3) Stable at high temperatures at which the ammoniatype stripper is unstable and to a lesser extent the cyanidetype stripper is also unstable.

(4) Being lower in cost of operation than the cyanidetype, ammonia-type and amine-type strippers; and

(5) Being typically nearly neutral or only slightly alkaline as contrasted with the higher alkalinity of the cyanide-, ammonia-, and amine-type strippers, and being free of handling problems and other problems attendant with handling of the stronger alkaline solutions.

The nature and composition of the disclosed baths is quite dissimilar to the commonly used formulations previously discussed. There is no cyanide or amine present. There is no free ammonia added, as in the case of the ammoniacal baths. However, ammonium salts including such salts of the carboxylic acid complexing agent herein, ammonium chloride, carbonate or sulfate, are used in our invention, and at the pH levels at which our baths are operated, some hydrolysis of the ammonium ion occurs. This results in a detectable odor of ammonia in the vapors immediately over the bath during operation. However, it is emphasized that this in no way makes the mechanism of operation of our bath similar or comparable to that of the ammoniacal bath. In the ammoniacal bath, the dissolved nickel is complexed by the ammonia present, probably as Ni(Nl-I as readily noted by the characteristic deep blue color. In our baths, on the other hand, the dissolved nickel produces the characteristic green color of nickel complexes with carboxylic acids. Thus, while some ammonia is present in our bath by bydrolysis, it plays only an incidental role, whereas in the ammoniacal bath it is the key to complexing of the nickel.

Description of the preferred embodiments The pH of the stripping baths herein should not be much below a pH of 6.8 to 7.0 inasmuch as at a pH of the bath much below such value, there occurs etching of the base metal of the object or article such as low carbon steel. The precise lower pH permissible to avoid the etching is a function of the temperature of the bath, with the graph of the accompanying drawing showing the approximate pH of the bath below which the steel is etched. The preferred pH of the bath at a bath temperature of about 220' C. is in the range of from about 7 to about 8.5. The pH values herein were determined electrometrically.

The upper pH range is determined by the point at which the nickel ceases to be complexed by carboxylic acids and becomes instead complexed by ammonia. The change can be readily seen on a visual basis, since the carboxylic complexes are green, and the ammoniacal complexes are blue. At the pH value at which the stripping bath changes color from green to blue, at least a major portion or predominant percentage of the nickel ions are being complexed by ammonia. Again, this change is not abrupt, but occurs over a pH range of about 0.3 units of pH. The pH at which the transition occurs is sharply temperature dependent because, in turn, the hydrolysis stability constant for ammonium ion in aqueous solution is sharply temperature dependent. The accompanying graph shows the pH levels as a function of temperature at which the transition occurs from a nickel-carboxylic acid complex to an ammoniacal complex. Further, at pH values above the upper pH limit or transition zone, as shown on the graph, brass and other copper alloys are etched by the stripper. The upper pH transition is sharper than the lower one. Therefore, for optimum selectivity, i.e. maximum differentiation between the rate of stripping of nickel and rate of attack on steel, copper and copper alloy substrates, it is most desirable to operate near the upper limit of the pH range. The line on the graph shown as l al 114*] is nearly such an optimum line.

The following are the recommended optimum pH values of the bath at the bath operating temperature set forth:

Temp. C pH (electrometn'c) 20 8.4

The data for the upper permissible pH range were calculated from the stability constants for the hydrolysis of ammonium ions in aqueous solutions, as recorded in Bjerrums textbook, Stability Constants:

mmi

K values for 50 C. are experimental values, as noted therein. K values for 50-80 C. were extrapolated using standard thermodynamic principles. A value of was arbitrarily decided upon as the upper value, beyond which the amount of free ammonia present can be said to constitute a gradual transformation from a solution in which ammonia is present primarily as the unhydrolyzed ammonium ion and nickel is primarily complexed by carboxylic acids to a solution which is primarily ammoniacal in nature. The K values used are as follows:

An attempt was made to correlate the above with visual observations of the solution. As shown in the accompanying graph, correlation is good to a temperature of 50 C. Beyond this temperature, visual readings of the transition point for nickel complexes run about 0.2 pH units higher in value than calculated from K values. Part of the problem may lie in certain experimental errors in obtaining high temperature pH measurements. Nonetheless, as the graph shows, visual determinations are approximately sufficient to determine the upper permissible pH range.

The graph may also be used to relate the pH of the claimed strippers at room temperature to pH values at operating temperatures. For example, as shown in the graph, if the initial pH at 20 C. is 8.4, it will be 7.4 at 55 C.

Even at values of NH /NH =0.1, there will be a gradual loss of ammonia, especially at elevated temperatures due to its significant partial vapor pressure. This will cause the pH of the working solution to fall during use. It should be restored by addition of the ammonium salts previously described and not by addition of ammonia itself. In making such pH adjustments, the pH may be determined either at operating temperatures of the bath, or at room temperature of the bath, using the graph to make the desired adjustment for the pH at operating temperature.

The bath temperature is preferably from room temperature to about 82 C. At temperatures much above 82 C., ammonium salts hydrolyze rather rapidly, evolving considerable ammonia, and thereby changing the characteristics of the bath so that it no longer operates according to the baths of this invention. In operation the most preferred bath temperatures are about 71 C. C.

The nitro-substitutcd aromatic compound is preferably present in the stripping bath or solution herein in amount from about 4 ounces to about 16 ounces per gallon, the carboxylic acid complexing agent or its water-soluble salt preferably in amount from about 6 ounces to about 18 ounces per gallon, the ammonium ion preferably in amount from about 2 ounces to about 8 ounces per gallon supplied as a water-soluble ammonium salt, and the sulfur compound preferably in small amount sufiicient to accelerate the nickel stripping up to about 0.5 ounce per gallon, more preferably from about 0.01 to about 0.5 ounce per gallon. By per gallon with regard each such ingredient is meant the amount of each ingredient stated for each one gallon of water. By ounces or ounce per gallon as used herein in specifying the preferred proportions of the ingredients of the stripping bath is meant the weight measure of the ingredients, as contrasted with fluid ounces which are a volumetric measurc. Wetting agents, for instance those of the anionic type, for example alkyl aryl sulfonates, may optionally be present in the bath solution in small amounts, typically from about .05-0.S ounce per gallon of water. Supplemental inhibitors may optionally be present in the bath solution for inhibiting attack on steel, such as watersoluble alkaline borates in a small amount up to about 0.5 ounce per gallon of water. Exemplary of the alkyl aryl sulfonates is methylnaphthalene sodium sulfonate. However, any bath-compatible wetting agent is utilizable in the stripping baths of this invention. Exemplary of the soluble borates is borax.

Exemplary of the complexing agents herein are adipic, citric, gluconic, glycolic, lactic, malic, succinic and tartaric acids, and their bath-compatible water-soluble salts, for instance alkaline salts, e.g. alkali metal, for example sodium or potassium, or ammonium salts of such acids. The acids set forth supra are preferred complexers of this invention.

By bath compatible as used herein in referring to the wetting agent and the Water-soluble salts of the carboxylic acid complexing agents is meant a wetting agent or such a salt which is non-detrimental to the other ingredients of the stripping bath and is present in a harmonious relationship with the other ingredients of the bath so that the bath can function in accordance with its intended purpose of stripping nickel from base metal substrates.

Exemplary of the sulfur compounds yielding in the aqueous stripping solution sulfur ions in a -2 oxidation state are metal sulfides, such as sodium, potassium, lithium, calcium, strontium or barium sulfide; aliphatic nitrogen sulfur compounds, such as thioureas which can be thiourea per se, diethyl thiourea and dibutyl thiourea, thiocarbamates such as sodium diethyl dithiocarbamate and dithane, thioamines such as monoethylthiourea and monomethylthiourea, thioamides such as thioacetarnide and thiopropionamide, and thiuram sulfides such as tetramethyl thiuram disulfide; aromatic nitrogen sulfur compounds such as mercaptobenzothiazoles which can be mercaptobenzothiazole per se, and thiazyls such as benzothiazyl disulfide and di-isopropyl-benzothiazyl-2-sulfenamide; thioacids and their salts such as thioorganic acids which can be thiomalic acid, thioacetic acid and thioglycollic acid, thiocyanates such as ammonium, sodium and potassium thiocyanate, and thiosulfates such as ammonium, sodium and potassium thiosulfate; non-metallic sulfides such as carbon disulfides, and sulfur halides such as S Cl SCl and SBr Preferred among such sulfur compounds for use herein are thiourea, sodium diethyldithiocarbamate, mercaptobenzothiazole, ammonium thiocyamate and ammonium thiosulfate. In addition to accelerating the stripping of nickel, these sulfur compounds prohibit the attack of the stripper solution on copper, brass and other copper base alloys.

Any of the nitro group substituted mononuclear carbocyclic aromatic compounds previously disclosed herein in the Description of the prior art subsection can be utilized in the stripping compositions herein. Preferred among such nitro-substituted aromatic compounds herein are sodium meta-nitrobenzenesulfonate and sodium metanitrobenzoate.

In a further embodiment, the invention also resides in dry stripper compositions which are well suited for dissolution in water to form the nickel-stripping baths. This dry composition, which is free of water except for that normally present in ingredients of the composition, comprises a mixture of from about 13 percent to about 64 percent of the nitro group-substituted mononuclear carbocyclic aromatic compound herein, from about 20 percent to about 72 percent of the complexing agent for nickel ions which is the carboxylic acid herein, or the water-soluble salt thereof, from about 8 percent to about 44 percent of the water-soluble ammonium salt herein and a sulfur compound providing or yielding in aqueous solution sulfur ions in a 2 oxidation state, present in small amount, suflicient to accelerate the nickel stripping, but not greater than about percent of the total composition, preferably from about 0.02 to 4 percent of this sulfur compound. All percentages of ingredients of the dry stripper composition herein are by weight and based on the total dry composition.

It was found that the sulfur compounds herein accelerated appreciably the stripping of nickel deposits which were free or substantially free of sulfur, but had little, if any, effect in accelerating the stripping of nickel deposits containing appreciable amounts of sulfur.

The method of this invention for stripping nickel from the base metal object or article involves contacting the nickel deposit on the object or article with the stripper aqueous solution, usually by immersing the base metal object having the nickel deposited or electroplated thereon in the stripper bath solution. The contacting of the nickel on the base metal object with the stripper solution is continued until the nickel is selectively dissolved from the base metal. Visual examination of the base metal object or article will show when the nickel has been completely or partially (if that is desired) stripped from the base metal. The base metal object is then separated from the stripper solution with the base metal of the object substantially unaltered by the solution.

The following examples are illustrative of nickel stripping baths of this invention without being restrictive:

EXAMPLE 1 Ounces per gallon of water Nitro group-substituted mononuclear carbocyclic aromatic compound herein 5 /2 Ammonium salt of a carboxylic acid herein 9 /2.

Sulfurcompound herein yielding in the aqueous stripping bath sulfur ions in a -2 oxidation state Ms Alkyl aryl sulfonate wetting agent /6 Alkaline borate herein A The above ingredients were dissolved in water in the proportions set forth and the thus-prepared bath was then ready for utilization in stripping nickel from base metals such as, for instance, steel, stainless steel, copper, magnesium, brass and alloys thereof. The temperature of the bath during stripping is preferably maintained at about 82 C. This stripping bath stripped nickel from the base metal at a rate of about 1 mil of nickel per hour. The pH of the bath is maintained within an especially preferred range of about 7.8-8.2 at room temperature of bath and at a pH of about 6.8-6.9 at bath temperature of about 82 C. by addition to the bath, as required, of ammonium carbonate.

EXAMPLE 2 Ounces per gallon of water Nitro group-substituted mononuclear carbocyclic aromatic compound herein 11 Ammonium salt of a carboxylic acid herein 19 Sulfur compound herein yielding in the aqueous stripping bath sulfur ions in a 2 oxidation state /3 Alkyl aryl sulfonate wetting agent /3 Alkaline borate herein 1 /2 The above ingredients were dissolved in water in the proportions specified, and the thus-prepared bath is utilizable for stripping nickel from any of the base metals disclosed in Example 1. At a preferred bath stripping temperature of about 81 C., the bath stripped nickel from the base metal at a rate of about 1.9 mil per hour. The pH of the bath is maintained within an especially preferred range of about 7.8-8.2 at room temperature of the bath, and at a pH of about 6.8-6.9 at bath temperature of about 81 C. by addition to the bath, as required, of ammonium carbonate.

As a convenience in bath makeup and replenishing, the stripper composition herein is preferred as a powdered solid mixture which is dry except for moisture normally present in its ingredients. The following such powdered dry mixture yielded substantially the bath of foregoing Example 1 when dissolved in water at 1 pound per gallon and the bath of foregoing Example 2 when dissolved in water at 2 pounds per gallon.

Percent by weight Nitro group-substituted mononuclear carbocyclic aromatic compound herein 34 Ammonium salt of a carboxylic acid herein 59 Sulfur compound herein yielding in the aqueous stripping bath sulfur ions in a 2 oxidation state 1 Alkyl aryl sulfonate wetting agent 1 Alkaline borate herein 5 EXAMPLE 3 A nickel stripping bath was prepared of the formulation and in accordance with Example 1 wherein the nitro group-substituted carbocyclic aromatic compound in this example was sodium meta-nitrobenzenesulfonate, the ammonium salt of the carbocyclic acid was ammonium citrate, the sulfur compound was ammonium thiocyanate, the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate and the alkaline borate was borax.

1 1 EXAMPLE 4 A nickel stripping bath was prepared of the formulation and in accordance with Example 2 wherein the nitro group-substituted carbocyclic aromatic compound in this example was sodium meta-nitrobenzoate, the ammonium salt of the carboxylic acid was ammonium citrate, the sulfur compound was ammonium thiocyanate, the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate, and the alkaline borate was borax.

EXAMPLE 5 A nickel stripping bath was prepared of the formulation and in accordance with Example 1 wherein the nitro group-substituted carbocyclic aromatic compound in this example was sodium meta-nitrobenzenesulfonate, the am monium salt of the carboxylic acid was ammonium adipate, the sulfur compound was thiourea, the alkyl aryl sulfonate was methylnaphthalene sodium sulfate and the alkaline borate was borax.

EXAMPLE 6 A nickel stripping bath was prepared of the formulation and in accordance with Example 2 wherein the nitro group-substituted carbocyclic aromatic compound was sodium meta-nitrobenzenesulfonate, the ammonium salt of the carboxylic acid was ammonium citrate, the sulfur compound was thiourea, the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate, and the alkaline borate was borax.

EXAMPLE 7 A nickel stripping bath was prepared of the formulation and in accordance with Example 2 wherein the nitro group-substituted carbocyclic aromatic compound was sodium meta-nitrobenzoate, the ammonium salt was ammonium gluconate, the sulfur compound was ammonium thiosulfate, the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate, and the alkaline borate was borax.

EXAMPLE 8 A nickel stripper bath was prepared of the formulation and in accordance with Example 1 wherein the nitro group-substituted carbocyclic aromatic compound was sodium meta-nitrobenzenesulfonate, the ammonium salt of the carboxylic acid was ammonium gluconate, the sulfur compound was ammonium thiocyanate, the alkaline borate was borax, and the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate.

EXAMPLE 9 A nickel stripper bath was prepared of the formulation and in accordance with Example 1 wherein the nitro group-substituted carbocyclic aromatic compound was sodium nitrobenzoate, the ammonium salt of the carboxylic acid was ammonium glycolate, the sulfur compound was ammonium thiocyanate, the alkaline borate was borax, and the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate.

EXAMPLE 10 A nickel stripper bath was prepared of the formulation and in accordance with Example 2 wherein the nitro group-substituted carbocyclic aromatic compound was sodium meta-nitrobenzene-sulfonate, the ammonium salt of the carboxylic acid was ammonium citrate, the sulfur compound was sodium diethyldithiocarbamate, the alkaline borate was borax, and the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate.

EXAMPLE 11 A nickel stn'pper bath was prepared of the formulation and in accordance with Example 2 wherein the nitro group-substituted carbocyclic aromatic compound was sodium meta-nitrobenzene sulfonate, the ammonium salt of the carboxylic acid was ammonium lactate, the sulfur compound was ammonium thiosulfate, the alkaline borate was borax, and the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate.

EXAMPLE 12 A nickel stripper bath was prepared of the formulation and in accordance with Example 2 wherein the nitro group-substituted carbocyclic aromatic compound was sodium meta-nitrobenzenesulfonate, the ammonium salt of the carboxylic acid was ammonium citrate, the sulfur compound was sodium thiosulfate, the alkaline borate was borax, and the alkyl aryl sulfonate was mcthylnaphthalcne sodium sulfonate.

EXAMPLE 13 A nickel stripper bath was prepared of the formulation and in accordance with Example 2 wherein the nitro group-substituted carbocyclic aromatic compound was sodium meta-nitrobenzenesulfonate, the ammonium salt of the carboxylic acid was ammonium tartrate, the sulfur compound was ammonium thiocyanate, the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate, and the alkaline borate was borax.

EXAM PLE l 4 A nickel stripper bath was prepared of the formulation and in accordance with Example 2 wherein the nitro group-substituted carbocyclic aromatic compound was sodium meta-nitrobenzoate, the ammonium salt of the carboxylic acid was ammonium citrate, the sulfur compound was sodium thiocyanate, the alkyl aryl sulfonate was methylnaphthalene sodium sulfonate, and the alkaline borate was borax.

EXAMPLE l5 Ounces per gallon Ingredient: of water Sodium meta-nitrobenzenesulfonate 5 /2 Succinic acid 7 Ammonium carbonate 6 Ammonium thiocyanate /6 Borax W4 Methyl naphthalene sodium sulfonate 16 The preferred operating temperature of the bath of this example is about 82 C., and the pH of the bath is maintained within an especially preferred range of about 7.8-8.2 at room temperature of the bath and at a pH of about 6.8-6.9 at the bath temperature of about 82 C. by addition to the bath, as required, of ammonium carbonate.

EXAMPLE 16 Ounces per Ingredient: gallon of water Sodium meta-nitrobenzenesulfonate 11 Succinic acid 15 Ammonium carbonate l2 Ammonium thio cyanate /3 Borax 1 /1 Methyl naphthalene sodium sulfonate A Methyl naphthalene sodium sulfonate The preferred operating temperature of the bath of this example is about 82 C., and the pH of the bath is maintained within an especially preferred range of about 7.8-8.2 at room temperature of the bath and at a pH of about 6.8-6.9 at the bath temperature of about 82 C. by addition to the bath, as required, of ammonium carbonate.

EXAMPLE 18 Ounces per Ingredient: gallon of water Sodium meta-nitrobenzenesulfonate 16 Citric acid 15 Ammonium carbonate 11 Ammonium thio cyanate /3 Borax 1 /2 Methyl naphthalene sodium sulfonate /3 The preferred operating temperature of the bath of this example is about 82 C., and the pH of the bath is maintained within an especially preferred range of about 78-82 at room temperature of the bath and at a pH of about 6.8-6.9 at the bath temperature of about 82 C. by addition to the bath, as required, of ammonium carbonate.

The following powdered mixture will yield approximately the bath of Example 17 when dissolved at 22 ounces per gallon of water, and that of Example 18 when dissolved at 44 ounces per gallon of water.

The preferred operating temperature of the bath of this example is about 82 C., and the pH of the bath is maintained within an especially preferred range of about 7.8-8.2 at room temperature of the bath and at a pH of about 6.8-6.9 at the bath temperature of about 82 C. by addition to the bath, as required, of ammonium carbonate.

EXAMPLE 20 Ounces per Ingredient: gallon of water Sodium meta-nitrobenzenesulfonate 7 Sodium citrate 27 Ammonium carbonate 11 Thiourea /3 Borax 1 /2 14 The preferred operating temperature of the bath of this example is about 82 C., and the pH of the bath is maintained within an especially preferred range of about 7.8-8.2 at room temperature of the bath and at a pH of about 6.8-6.9 at the bath temperature of about 82 C. by addition to the bath, as required, of ammonium carbonate. The following powder mixture will yield approximately the bath of Example 19 when dissolved at 24 ounces per gallon of water, and that of Example 20 when dissolved at 48 ounces per gallon of water.

Percent by weight Sodium meta-nitrobenzenesulfonate 15 Sodium citrate 57.5 Ammonium carbonate 23.5 Thiourea 1 Borax 3 Inasmuch as the present invention employs neither cyanides nor amines, it is clearly not related to the cyanide-type and amine-type nickel strippers, the most commonly used today. It diifers widely in principle from the ammoniacal stripper even though some ammonia is given off by the baths of our invention through incidental decomposition.

Ammoniacal strippers contain significant amounts of both ammonium salts and of ammonia. The baths of our invention contain significant amounts of ammonium salts only, and not any significant amount of ammonia. This results in a profound difference in the chemical principles which govern the performance of the two baths. In the case of the ammoniacal stripper, nickel ion is complexed by ammonia, Ni(NH a blue complex and resulting in a characteristic blue color of the stripping solution or bath. Thus continuing loss of ammonia, unavoidable at the high temperatures and high alkalinities of the ammoniacal stripper, results in severe imbalance of a key ingredient of the bath. This imbalancing results either in markedly reduced nickel stripping rates, or severe pitting and/or etching of steel substrates, or both. By way of sharp contrast, in the baths of our invention, nickel ion is complexed by the carboxylic acids of this invention, as evidenced by the characteristic green color of the stripping solution. Thus, the incidental loss of small amounts of ammonia by volatilization in no way affects the performance of the bath, either with respect to rate of nickel stripping or to etching of steel, copper or copper alloy substrates provided the pH is properly maintained.

Although certain preferred embodiments of the invention have been elaborated upon herein, it is understood that such embodiments specified, purely for purposes of illustration, should not restrictthe breadth and scope of the invention as defined by the accompanying claims.

What is claimed is:

1. A bath for stripping nickel deposited on a base metal object consisting essentially of an aqueous solution of:

(1) a nitrosubstituted mononuclear carbocyclic aromatic compound having at least one --NO group attached to a benzene ring and a water-solubilizing substituent also attached to the benzene ring present in an amount from about 4 ounces to about 16 ounces per gallon;

(2) a complexing agent for nickel ions selected from the group consisting of (a) a tribasic carboxylic acid of the formula wherein R and R are selected from the class consisting of CI -l and -CHOH- and R is selected from the group consisting of and water-soluble salts thereof, (b) a dibasic carboxylic acid of formula HOOC(CH -COOH wherein R is selected from the group consisting of -H and COOH, R is selected from the group consisting of -CH and CHOH-, and n is an integer from to 4 inclusive, and water-soluble salts thereof, and (d) mixtures of at least two of (a), (b) and (c), the complexing agent being present in an amount of from 6 ounces to about 18 ounces per gallon;

(3) ammonium ions supplied as a water-soluble ammonium salt selected from the group consisting of the ammonium salts of the carboxylic acids of paragraph (2) and ammonium chloride, carbonate and sulfate in amount sufiicient to maintain the pH of the bath within the range from about 6.8-7.0 to up to but below a predetermined higher pH value at which nickel ions substantially cease to become complexed by the carboxylic acid and instead are ccrnplexed by ammonia as indicated by a change in coloration of the bath from green to blue; and

(4) a sulfur compound yielding in the aqueous solution sulfur ions in a 2 oxidation state; being present in amount from about 0.01 to about 0.5 ounces per gallon, the pH of the bath being within the pH range aforesaid.

2. The bath of claim 2 wherein the ammonium ion is present in amount from about 2 ounces to about 8 ounces per gallon, supplied as the water-soluble ammonium salt.

3. The bath of claim 2 wherein the nitro-substituted aromatic compound is selected from the group consisting of sodium meta-nitrobenzenesulfonate and sodium metanitrobenzoate; the complexiug agent is selected from the group consisting of adipic, citric, gluconic, glycolic, lactic, malic, succinic and tartaric acids, and water-soluble salts thereof; and the sulfur compound is selected from the group consisting of thiourea, sodium diethyldithiocarbamate, mercaptobenzothiazole, ammonium thiocyanate and ammonium thiosulfate.

4. The bath of claim 2 wherein the weight ratio of the nitro-substituted aromatic compound to the carboxylic acid is between about 1 and about 1 to 2 respectively.

5. A method for stripping nickel deposited on a base metal object from the base metal, which comprises:

(A) contacting the nickel deposit on the base metal object with an aqueous solution consisting essentially of:

(1) a nitro-substituted mononuclear carbocyclic aromatic compound having at least one -NO group attached to a benzene ring and a watersolubilizing substituent also attached to the her:- zene ring present in an amount from about 4 ounces to about 16 ounces per gallon;

(2) a complexing agent for nickel ions selected from the group consisting of (c) a tribasie carboxylic acid of the formula wherein R and R are selected from the class consisting of CH and CHOH- and R is selected from the group consisting of and water-soluble salts thereof, (c) a-hydroxy monoand di-carboxylic acids of the formula R (R CHOHCOOI-I 16 wherein R is selected from the group consisting of --H and COOH, R is selected from the group consisting of CH';-- and and n is an integer from 0 to 4 inclusive, and water-soluble salts thereof, and (d) mixtures of at least two of (a), (b) and (c), the complexing agent being present in an amount of from about 6 ounces to about 18 ounces per gallon;

(3) ammonium ions supplied as a water-soluble ammonium salt selected from the group consisting of the ammonium salts of the carboxylic acids of paragraph (2) and ammonium chloride, carbonate and sulfate in amount suflicient to maintain the pH of the bath within the range from about 6.8-7.0 to up to but below a predetermined higher pH value at which nickel ions substantially cease to be complexed by the carboxylic acid and instead are complexed by ammonia as indicated by a change in coloration of the bath from green to blue; and

(4) a sulfur compound yielding in the aqueous solution sulfur ions in a --2 oxidation state; being present in amount from about 0.01 to about 0.5 ounces per gallon, the pH of the bath being within the pH range aforesaid; and

(B) continuing the contacting of the nickel on the base metal object with said aqueous solution until the nickel is selectively dissolved from the base metal object.

6. The method of claim 5 wherein the base metal of the object is selected from the group consisting of steel, copper and copper base alloys.

7. The method of claim 5 wherein the stripper aqueous solution is at a temperature in the range of from room temperature to about 82 C.

8. The method of claim 5 wherein the stripper aqueous solution is maintained at a pH in the range from about 7 to about 8.5 at a stripper solution temperature of about 20 C.

9. A dry composition suitable for dissolution in water to form a nickel-stripping bath consisting essentially of:

(1) a nitro-substituted mononuclear carbocyclic aromatic compound having at least one -NO group attached to a benzene ring and a water-solubilizing substituent also attached to the benzene ring, in amount from about 13 percent to about 64 percent; (2) a complexing agent for nickel ions selected from the group consisting of (a) a tribasic carboxylic acid of the formula wherein R and R are selected from the group consisting of CH and -CHOH- and R is selected from the group consisting of and water-soluble salts thereof, (b) a dibasic carboxylic acid of the formula HOOC(CH --COOH wherein n is an integer from 1 to 4 inclusive and water-soluble salts thereof, (0) a-hydroxy monoand di-carboxylic acids of the formula wherein R is selected from the group consisting of H and COOH, R is selected from the group consisting of -CH and CHOH-- and n is an integer from 0 to 4 inclusive, and water-soluble salts thereof, and (d) mixtures of at least two of (a), (b)

and (c), in amount from about 20 percent to about 72 percent;

(3) a water-soluble ammonium salt in amount from about 8 percent to about 44 percent; and

(4) a sulfur compound capable of providing in aqueous solution sulfur ions in a -2 oxidation state, present in a small amount, suflicient to accelerate stripping of nickel but not greater than about 5 percent of the total composition, all percentages being by weight and based on the total composition.

10. The composition of claim 9 wherein the nitrosubstituted mononuclear carbocyclic aromatic compound is selected from the group consisting of sodium metanitrobenzienesulfonate and sodium meta-nitrobenzoate, the complexing agent is selected from the group consisting of adipic, citric, gluconic, glycolic, lactic, malic, succinic and tartaric acids, and water-soluble salts thereof; the

soluble ammonium salt is selected from the group consisting of soluble ammonium salts of the aforementioned complexing agent carboxylic acids and the chloride, carbonate and sulfate of ammonium; and the sulfur compound is selected from the group consisting of th-iourea, sodium diethyldithiocarbamate, mercaptobenzothiazole, ammonium thiocyanate and ammonium thiosulfate.

References Cited UNITED STATES PATENTS 2,649,361 8/1953 Springer et a1. 156--18 2,698,781 1/1955 Meyer 15618 2,937,940 5/1960 Weisberg et a1 252-149 3,102,808 9/1963 Weisberg et a1. 252-449 LEON D. ROSDOL, Primary Examiner.

WILLIAM E. SCHULZ, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5,365,401

January 23, 1968 Edward B. Saubestre et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 54, for

column 13, line 55, for "metametanitrobenzenesulfonate "from" insert about Signed and sealed this (SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer 1st day of April 1969.

EDWARD J. BRENNER Commissioner of Patents 

